Lithium ion batteries are presently anticipated as a useful power source due to growing consciousness to environmental problems in the worldwide scale and for realization of the low-carbon society. For example, as high-capacity chargeable and dischargeable batteries for a power storage system, nonaqueous electrolyte solution-based lithium ion batteries using a large-sized laminate cell have been used practically. Lithium ion batteries are anticipated also as batteries for electric vehicles (EVs) and hybrid electric vehicles (HEVs). Particularly lithium ion batteries mounted on electric vehicles conceivably need long-term life properties over at least 10 years. Therefore, the lithium ion batteries are required to have a low capacity degradation rate to the number of times of charge and discharge, and further absolutely not to cause a rapid capacity degradation phenomenon, the so-called rapid fading phenomenon, while the vehicle is driven, in order to enhance the safety. The lithium ion batteries are also required to have a resistance as the whole battery as low as possible in order to be operated even in a low-temperature environment.
A lithium ion battery is usually constituted of a positive electrode, a negative electrode, an electrolyte, and a separator. As a positive electrode active material to be used for the positive electrode, lithium cobaltate (LiCoO2), manganese spinel (LiMn2O4), and the like are mainly used. Since the positive electrode active material has a high electric resistance, the electric resistance of the positive electrode is decreased by using carbon-based conductive additives. As a binder, for example, styrene-butadiene rubber, fluororubber, synthetic rubber, a polymer such as polyvinylidene fluoride, an acryl resin, and the like are used.
A negative electrode active material to be used is natural graphite, artificial graphite obtained by thermally treating coal, petroleum pitch or the like at a high temperature, amorphous carbon obtained by thermally treating coal, petroleum pitch coke, acetylene pitch coke or the like, a lithium alloy such as metallic lithium or AlLi, or the like. Carbon-based conductive additives are used for a negative electrode in some cases for the purpose of decreasing the resistance.
As an electrolyte solution, a nonaqueous electrolyte solution in which an electrolyte such as a lithium salt is dissolved, is used. As the lithium salt, LiPF6, LiBF4, a lithium imide salt, LiClO4, or the like are used. A separator is constituted of a film to separate a positive electrode and a negative electrode and prevent short-circuit of both the electrodes.
In lithium ion batteries having the above-mentioned constitutions, a technology using a spherical or massive carbon-based material as a negative electrode active material is described in Patent Literatures 1 and 2 (JP11-154513A and JP11-263612A). When a negative electrode active material is made in such a shape, a crystal orientation of the negative electrode active material is directed in various directions even after a rolling step for negative electrode fabrication. Thereby, lithium ions transfer smoothly between electrodes, and a lithium ion battery excellent in output properties can be made. In addition, there are many gaps are between the negative electrode active materials, and flow paths of an electrolyte solution are formed also in the direction perpendicular to the thickness direction of the negative electrode, thereby contributing smooth transfer of lithium ions. Therefore, in many lithium ion batteries for EVs and HEVs, as the negative electrode active material, a spherical or massive carbon-based material comes to be used.
However, on the other hand, if a negative electrode active material is made of a spherical or massive shape, the contact between the negative electrode active materials is liable to become point contact. Therefore, the electric resistance (electronic resistance) of a conductive network to carry electrons to a collector sometimes becomes high and sometimes become instable. Then, in order to reduce the electric resistance of a negative electrode, Patent Literature 3 (JP2005-142004A) discloses a technology of adding conductive additives of carbon blacks. Since carbon black is composed of primary particles of the order of several tens of nanometers, carbon black is easy to aggregate, and carbon black forms secondary particles and bridges between active materials; therefore, the carbon black is effective for securing the conductivity in the early charge and discharge cycle.